This invention relates to components for a semi-conductor diffusion furnace.
U.S. Pat. No. 3,951,587 discloses semi-conductor diffusion furnace components such as a process tube, paddle and boat consisting essentially of a sintered silicon carbide matrix resulting from the sintering of silicon carbide powder which is made up of approximately 50% by weight of fine silicon carbide having an average particle size of 0.1-8 microns and 50% by weight of coarse silicon carbide having an average particle size of 30-170 microns.
It is described in U.S. Pat. No. 3,951,587 that original silicon carbide powder may be all fine or all coarse or any mixture thereof other than the preferred 50% 0.1 to 8 microns and 50% 30 to 170 micron combination. However, if for example the powder is all of the finer size, then the sintered structure may lack sufficient continuous porosity to permit complete impregnation of the silicon carbide matrix with silicon. The silicon would essentially only impregnate the surface pores or irregularities so that only partial impregnation would occur. On the other hand if the silicon carbide powder is substantially coarser, the porosity would be more than adequate in size, as would be the degree of continuity between the pores, but the sintered structure would have lower mechanical strength properties than when the bimodal approach is employed. Accordingly, the bimodal approach is advantageous.
However, it is very difficult to reduce in size the silicon carbide powder, for example, by grinding or milling, because silicon carbide is extremely hard.
Also, fine silicon carbide powder has very large surface area as a whole because the average particle size thereof is 0.1-8 microns. When the coarse silicon carbide is ground into the fine silicon carbide, impurities are apt to be mixed therewith.
In order to avoid such impurities, a special process is additionally required, which increases production costs.